Development of neuropsychological theory is facilitated when there is method and theory to interrelate nonhuman animal and human studies. One means by which such interrelations might be developed, to help clarify the nature of information processing in the hippocampal system, consists of combining method and theory articulated by Ashby and his colleagues in the arena of human cognition and decision making (Ashby & Gott, 1988) with a hippocampal processing theory developed by Eichenbaum and his colleagues (Eichenbaum, 1994). Eichenbaum and his colleagues suggest that the hippocampus is responsible for flexible, relational processing while Ashby and his colleagues have developed a categorization task for use in humans where a subject learns to optimally categorize stimulus displays based on a relation between features in the displays. There are few direct tests of the theory developed by Eichenbaum and his colleagues. Therefore, the aim of the proposed research is to assess the circumstances under which hippocampal processing is required for normal performance in the Ashby categorization task.